Electron emitter and method of fabricating same

ABSTRACT

Electron emitters and methods of fabricating the electron emitters are disclosed. According to certain embodiments, an electron emitter includes a tip with a planar region having a diameter in a range of approximately (0.05-10) micrometers. The electron emitter tip is configured to release field emission electrons. The electron emitter further includes a work-function-lowering material coated on the tip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 17/022,504,filed Sep. 16, 2020, which is a continuation of application Ser. No.16/324,518, filed Feb. 8, 2019, now U.S. Pat. No. 10/784,071, which is aNational Stage of PCT/EP2017/069270, filed Jul. 31, 2017, which claimspriority of U.S. application 62/372,084, filed on Aug. 8, 2016 and U.S.application 62/531,793, filed on Jul. 12, 2017, the contents of all ofwhich are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure generally relates to the field ofcharged-particle sources, and more particularly, to electron emittersused in electron-beam apparatuses and methods of fabricating theelectron emitters.

BACKGROUND

In manufacturing processes of integrated circuits (ICs), unfinished orfinished circuit components are inspected to ensure that they aremanufactured according to design and are free of defects. An inspectionsystem utilizing an optical microscope typically has resolution down toa few hundred nanometers; and the resolution is limited by thewavelength of light. As the physical sizes of IC components continue toreduce down to a sub-100 or even sub-10 nanometers, inspection systemscapable of higher resolution than those utilizing optical microscopesare needed.

A charged particle (e.g., electron) beam microscope, such as a scanningelectron microscope (SEM) or a transmission electron microscope (TEM),capable of resolution down to less than a nanometer, serves as apracticable tool for inspecting IC components having a feature size thatis sub-100 nanometers. With a SEM, electrons of a single primaryelectron beam, or electrons of a plurality of primary electron beams,can be focused at predetermined scan locations of a wafer underinspection. The primary electrons interact with the wafer and may bebackscattered or may cause the wafer to emit secondary electrons. Theintensity of the electron beams comprising the backscattered electronsand the secondary electrons may vary based on the properties of theinternal and/or external structures of the wafer, and thus indicateswhether the wafer has defects.

However, due to their high resolution, typical electron-beam inspectiontools have low throughput. This limits the electron-beam inspectiontools from being applied to wafer inspection in large scale. One way toimprove the throughput is to increase the beam current of the primaryelectron beam, such that it can scan a larger area on the wafer, or itcan be split into multiple beamlets for scanning multiple separate areassimultaneously. Current electron emitters, such as Schottky emitters,although capable of generating bright illumination, only have a smallemission area from which electrons can be emitted. This limits themaximum beam current achievable at a given brightness. Moreover, theemission area may easily get deformed during high-brightness operation(i.e., high temperature and/or high electric field), which causesinstabilities to the emitted electron beam(s) or reduces the emissionarea. Thus, current electron emitters cannot meet the high-throughputrequirement.

SUMMARY

Embodiments of the present disclosure relate to electron emitters andmethods of fabricating the electron emitters. In some embodiments, anelectron emitter is provided. The electron emitter includes a tip with aplanar region having a diameter in a range of approximately (0.05-10)micrometers. The electron emitter further includes awork-function-lowering material coated on the tip.

In some embodiments, a thermal field emission cathode is provided. Thethermal field emission cathode includes an emitter, which furtherincludes a tip configured to release field emission electrons, the tiphaving a planar region with a diameter in a range of approximately(0.05-10) micrometers. The thermal field emission cathode also includesa work-function-lowing material coated on the tip. The thermal fieldemission cathode further includes a heating component configured toprovide thermal energy to the emitter.

In some embodiments, a method of fabricating an electron emitter isprovided. The method includes applying a restraint to an electronemitter having a tip. The method also includes, under the restraint,forming a planar region on the tip. The method further includes removingthe restraint.

In some embodiments, a method of fabricating an electron emitter isprovided. The method includes coating a work-function-lowering materialon a tip of an electron emitter having a base material. Thework-function-lowering material includes at least one of: an oxidecompound of zirconium, hafnium, titanium, scandium, yttrium, vanadium,lanthanum, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, ytterbium, lutetium, of thorium; anitride compound of zirconium, titanium, niobium, scandium, vanadium, orlanthanum; and an oxynitride compound of zirconium, hafnium, titanium,scandium, yttrium, vanadium, lanthanum, praseodymium, neodymium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,ytterbium, lutetium, niobium, or thorium.

Additional objects and advantages of the disclosed embodiments will beset forth in part in the following description, and in part will beapparent from the description, or may be learned by practice of theembodiments. The objects and advantages of the disclosed embodiments maybe realized and attained by the elements and combinations set forth inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosed embodiments, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary electron beamtool, consistent with embodiments of the present disclosure.

FIG. 2 is a schematic diagram illustrating an exemplary thermal fieldemission cathode used in the exemplary electron beam tool of FIG. 1 .

FIG. 3A is a schematic diagram illustrating a conventional tungstenemitter tip without coating material.

FIG. 3B is a schematic diagram illustrating an emitter tip coated withwork-function-lowering material, consistent with embodiments of thepresent disclosure.

FIG. 4 is a flowchart of an exemplary method of preparing an electronemitter, consistent with embodiments of the present disclosure.

FIG. 5 is a flowchart of an exemplary method of preparing an electronemitter, consistent with embodiments of the present disclosure.

FIG. 6 is a schematic diagram illustrating a fixture or jig attached toan emitter tip of the thermal field emission cathode of FIG. 2 ,consistent with embodiments of the present disclosure.

FIG. 7 is a flowchart of an exemplary method of preparing an electronemitter, consistent with embodiments of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses and methods consistent with aspects related tothe invention as recited in the appended claims.

The present application discloses electron emitters that can generateelectron beams with high brightness and beam current, as well as methodsfor preparing or fabricating the electron emitters. The disclosedelectron emitters may be used in many technologies, such as inmanufacturing processes of integrated circuits (ICs). FIG. 1 is aschematic diagram illustrating an exemplary electron beam tool 10,consistent with embodiments of the present disclosure. As shown in FIG.1 , electron beam tool 10 includes a motorized stage 134 and a waferholder 136 supported by motorized stage 134 to hold a wafer 150 to beinspected. Electron beam tool 10 further includes a cathode 100, ananode 120, a gun aperture 122, a beam limit aperture 124, a condenserlens 126, a source conversion unit 128, an objective lens assembly 132,a beam separator 138, and an electron detector 140. Source conversionunit 128, in some embodiments, can include a micro-deflectors array 129and a beamlet-limit plate 130. Objective lens assembly 132, in oneembodiment, can include a modified swing objective retarding immersionlens (SORIL), which includes a pole piece 132 a, a control electrode 132b, a deflector 132 c, and an exciting coil 132 d. Electron beam tool 10may additionally include an energy dispersive X-ray spectrometer (EDS)detector (not shown) to characterize the materials on the wafer.

When electron beam tool 10 operates, a wafer 150 to be inspected ismounted or placed on wafer holder 136, which is supported by motorizedstage 134. A voltage is applied between anode 120 and cathode 100,cathode 100 emits an electron beam 160. The emitted electron beam passesthrough gun aperture 122 and beam limit aperture 124, both of which candetermine the size of electron beam entering condenser lens 126, whichresides below beam limit aperture 124. Condenser lens 126 can focus theemitted electron beam 160 before electron beam 160 enters sourceconversion unit 128. Micro-deflectors array 129 can split the emittedbeam into multiple primary electron beams 160 a, 160 b, and 160 c. Thenumber of multiple primary beams is not limited to three andmicro-deflector array 129 can be configured to split the emitted beaminto greater number of primary electron beams. Beamlet-limit plate 130can set the size of the multiple primary electron beams before enteringobjective lens assembly 132. Deflector 132 c deflects the primaryelectron beams 160 a, 160 b, and 160 c to facilitate beam scanning onthe wafer. For example, in a scanning process, deflector 132 c can becontrolled to deflect primary electron beams 160 a, 160 b, and 160 csimultaneously onto different locations of top surface of wafer 150 atdifferent time points, to provide data for image reconstruction fordifferent parts of wafer 150.

Exciting coil 132 d and pole piece 132 a generate a magnetic field thatbegins at one end of pole piece 132 a and terminates at the other end ofpole piece 132 a. A part of wafer 150 being scanned by primary electronbeam 160 can be immersed in the magnetic field and can be electricallycharged, which, in turn, creates an electric field. The electric fieldreduces the energy of impinging primary electron beam 160 near thesurface of the wafer before it collides with the wafer. Controlelectrode 132 b, being electrically isolated from pole piece 132 a,controls an electric field on the wafer to prevent micro-arching of thewafer and to ensure proper beam focus.

Backscattered primary electrons and secondary electrons can be emittedfrom the part of wafer 150 upon receiving primary electron beams 160 a,160 b, and 160 c. Beam separator 138 can direct the secondary and/orscattered electron beams 170 a, 170 b, and 170 c, comprisingbackscattered and secondary electrons, to sensor surface of electrondetector 140. The detected electron beams 170 a, 170 b, and 170 c canform corresponding beam spots 180 a, 180 b, and 180 c on the sensorsurface of electron detector 140. Electron detector 140 can generatesignals (e.g., voltages, currents, etc.) that represent the intensitiesof the received beam spots, and provide the signals to a processingsystem (not shown in FIG. 1 ). The intensity of secondary and/orscattered electron beams 170 a, 170 b, and 170 c, and the resultant beamspots, can vary according to the external and/or internal structure ofwafer 150. Moreover, as discussed above, primary electron beams 160 a,160 b, and 160 c can be projected onto different locations of the topsurface of wafer 150 to generate secondary and/or scattered electronbeams 170 a, 170 b, and 170 c (and the resultant beam spots) ofdifferent intensities. Therefore, by mapping the intensities of the beamspots with the locations of wafer 150, the processing system canreconstruct an image that reflects the internal and/or externalstructures of wafer 150.

Although FIG. 1 shows electron beam tool 10 as a multi-beam inspectiontool that employs multiple primary electron beamlets to simultaneouslyscan multiple locations on wafer 150, it is contemplated that electronbeam tool 10 may also be a single-beam inspection tool that uses onlyone primary electron beam to scan one location of wafer 150 at a time.Moreover, electron beam tool 10 may also be implemented as anelectron-beam lithography (EBL) system, such as an electron-beam directwrite (EBDW) system. The present application does not limit the specificsystem or technology area where the disclosed electron emitter isapplied.

Whether electron beam tool 10 is used to inspect a wafer or performelectron-beam lithography, the disclosed electron emitters can emitlarger beam current, so as to improve the throughput of electron beamtool 10. FIG. 2 is a schematic diagram illustrating a cathode 100 usedin the exemplary electron beam tool of FIG. 1 . In exemplaryembodiments, cathode 100 may be a thermal field emission cathode, e.g.,a Schottky cathode, which uses a combination of heat and electric fieldto emit electrons. Referring to FIG. 2 , cathode 100 includes anelectron emitter 102, a filament 110, two electrodes 112, and a base114.

Base 114 is made of an electrically insulating material, such as ceramicor thermal ceramic. In some embodiments, the electrically insulatingmaterial may be zirconium dioxide (ZrO₂), aluminum oxide (Al₂O₃),aluminum nitride (AlN), etc. Base 114 supports the two electrodes 112.Each electrode 112 has one end embedded in base 114. Electrodes 112 aremade of electrically conductive material, such as nickel ferrous alloy.

Filament 110 is an electrically conductive wire, made of, for example,tungsten or rhenium. The two ends of filament 110 are welded to twoelectrodes 112, respectively. Filament 110 may also be bended at acentral portion. The bended angle of filament 110 may range from 10 to100 degree. Electron emitter 102 is mounted on the central portion offilament 110, such that filament 110 is convex to electron emitter 102at the central portion.

Electron emitter 102 includes an emitter tip 104, which apex 106. Apex106 bray be a planar region. The emitted electrons are in a narrowenergy band and are emitted from apex 106 into a cone of emission.Normally, to escape from electron emitter 102, an electron must gainsufficient energy to overcome an energy barrier posed by the atomsand/or molecules present at the surface of apex 106. The amount ofenergy required to overcome the energy hairier is known as work functionof electron emitter 102. In exemplary embodiments, emitter tip 104 andparticularly spec 106 may be coated with a thin layer of coatingmaterial 108 to lower the work function. In the present disclosure, thematerial constituting the body of electron emitter 102 is referred to as“base material,” and the coating material 108 is referred to as“work-function towering material.”

When cathode 100 is implemented as a Schottky cathode, electric currentis supplied to filament 110 through electrodes 112. Filament 110 heatselectron emitter 102 and thermally excites the electrons in electronemitter 102 such that they can escape over the work-function barrier.Additionally or alternatively, cathode 100 and anode 120 may generate astrong electric field at emitter tip 112, which facilitates the emissionof electrons by tunneling through the work-function barrier. Byadjusting, the emitter temperature and/or strength of the electricfield, cathode 100 may change the beam current emitted from electronemitter 102.

Schottky cathodes are capable of generating bright electron beams.Emitters used in typical Schottky cathodes, i.e., typical Schottkyemitters, are made from a single crystal of tungsten oriented in the<100>, <110>, <111>, or <310> orientation. The Schottky emitters mayalso be made from other base material, such as molybdenum, iridium, orrhenium. The Schottky emitters may also be coated withwork-function-lowering material, including for example, compounds suchas oxide, nitride, and carbon compound of zirconium, titanium, hafnium,yttrium, vanadium, thorium, scandium, beryllium, or lanthanum. Forexample, by making a Schottky emitter's apex surface to be the (100)crystal plane of tungsten and using zirconium oxide (ZrO) as thework-function-lowering material, the work function of the Schottkyemitter may be lowered from 4.5 eV to 2.8 eV. The reduction of workfunction makes the Schottky emitter a brighter electron source. Theworking temperature of such ZrO coated tungsten emitter, i.e., thetemperature at the emitter's apex, is in the range of (300-1,800)K.

Although a tungsten Schottky emitter is capable of generating brightelectron beams, the electric field and temperature applied on theemitter may cause surface self-diffusion at the emitter's apex.Specifically, at the high working temperature of the Schottky emitter,the base material and coating material tends to evaporate from theemitter's apex, which changes the original planar surface of the apex toa curved surface. Meanwhile, the high electric field causes the basematerial and coating material at the apex to migrate and therefore thesurface of the apex to shrink, e.g., sharpening the emitter's tip. Assuch, the combined effect of high temperature and high electric fieldtends to cause the apex to have an irregular surface. FIG. 3A is aschematic diagram illustrating a tip 204 of a conventional tungstenemitter without coating material. As shown in FIG. 3A, tip 204 includesan apex 206, which originally has a planar surface (not shown). However,as the tungsten emitter keeps operating under high temperature and highelectric field, the surface of apex 206 gradually gets deformed, whichcauses instabilities to the electron emission and lowers the beamcurrent.

Referring back to FIG. 2 , in the disclosed embodiments, to reduce thesurface deformation, the base material of electron emitter 102 may bechosen from transition-metal-carbide compounds and/ortransition-metal-boride compounds. For example, thetransition-metal-carbide compound may be a carbide compound of hafnium,zirconium, tantalum, titanium, tungsten, molybdenum, or niobium. Alsofor example, the transition-metal-boride compound may be a boridecompound of hafnium, zirconium, tantalum, tungsten, molybdenum, niobium,or lanthanum.

Compared to tungsten, transition metal carbides or transition metalborides have higher melting points, higher hardness, and lower workfunctions. For example, hafnium carbide has a melting point of 4,163Kand has work functions in the (3.3-3.6)eV range. These properties oftransition metal carbides and transition metal nitrides make them lesssusceptible to surface deformation under high temperature and/or highelectric field.

In some embodiments, work-function-lowering, material may be coated ontransition metal carbides and transition metal borides, to further lowertheir work functions. The work-function-lowering material may include atleast one of an oxide compound of zirconium, hafnium, titanium,scandium, yttrium, vanadium, lanthanum, praseodymium, neodymium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,ytterbium, lutetium, or thorium.

Alternatively or additionally, the work-function-lowering material mayinclude at least one of a nitride compound of zirconium, titanium,niobium, scandium, vanadium, or lanthanum.

Alternatively or additionally, the work-function-lowering material mayinclude at least one of an oxynitride compound of zirconium, hafnium,titanium, scandium, yttrium, vanadium, lanthanum, praseodymium,neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium,erbium, ytterbium, lutetium, niobium, or thorium.

FIG. 3B is a schematic diagram illustrating an emitter tip 104 coatedwith work-function-lowering material 108, consistent with embodiments ofthe present disclosure. The base material of emitter tip 104 includestransition metal carbides and/or transition metal borides.Work-function-lowering material 108 includes the above-disclosedwork-function-lowering material and may lower the work functions oftransition metal carbides and/or transition metal borides, for example,from approximately 3.3 eV to approximately 2.3 eV. With the lowered workfunctions, the angular intensity of the electron beam emitted from apex106, defined as the electron current (i.e., beam current) divided by thesolid angle through which the electrons are emitted, may reach the rangeof (15-400)mA/Sr. Moreover, the electric field required to be applied onemitter tip 104 to achieve a given brightness may be reduced. Suchweaker electric field reduces the chance of surface deformation at apex106. As shown by FIG. 3B, the planar surface of apex 106 may bemaintained without deformation, for a prolonged time period.

In the disclosed embodiments, the emission area, i.e., the planar regionat apex 106, of electron emitter 102 may also be enlarged, to increasethe angular intensity of the electrons emitted from apex 106. Referringback to FIG. 2 , in some embodiments, apex 106 may be configured to havea diameter in a range of approximately (0.05-10) micrometers. Forexample, the angular intensity of the electron beam emitted from atungsten emitter with such apex diameter can reach the range of(1-25)mA/Sr.

Traditionally, it has been difficult to enlarge the size of the apex ofa tungsten tip because of the relative mechanical strength of tungsten.As explained above, transition metal carbides or transition metalborides have higher hardness than tungsten. Thus, compared to tungstenemitters, it is easier polish an emitter tip made from transition metalcarbides or transition metal borides. Moreover, methods 500 and 700described below may be used to enlarge the size of the apex of anemitter tip, whether the emitter tip is made from tungsten, transitionmetal carbides, transition metal borides, or other kinds of basematerial.

With increased angular intensity of the emitted electrons, disclosedelectron emitter 102 may help improve the throughput of electron beamtool 10. For example, when electron beam tool 10 is a single-beaminspection tool, a primary electron beam with higher angular intensitycan be used to scan a larger area on wafer 150 or to performvoltage-contrast (VC) defect inspection of high aspect ratio contacts(HARCs). As another example, when electron beam tool 10 is amultiple-beam inspection tool, the higher angular intensity makes itfeasible to divide the primary electron beam into multiple beamlets, sothat multiple locations on wafer 150 may be scanned simultaneously.Moreover, similar to the case of single-beam inspection tool, the higherangular intensity makes it possible for the multi-beam inspection toolto perform voltage-contrast defect inspection. As yet another example,when electron beam tool 10 is an EBDW system, the higher angularintensity provides larger beam current, which improves the lithographyefficiency.

Next, exemplary methods of preparing or fabricating the disclosedelectron emitters are disclosed. FIG. 4 is a flowchart of exemplarymethod 400 of preparing an electron emitter, according to someembodiments of the present disclosure. For example, the electron emittermay be electron emitter 102 shown in FIG. 2 . The base material of theelectron emitter may include at least one of a transition-metal-carbidecompound or a transition-metal-boride compound. Thetransition-metal-carbide compound may be a carbide compound of hafnium,zirconium, tantalum, titanium, tungsten, molybdenum, or niobium. Thetransition-metal-boride compound may be a boride compound of hafnium,zirconium, tantalum, titanium, tungsten, molybdenum, niobium, orlanthanum. Method 400 may be performed to coat a work-function-loweringmaterial (e.g., work-function-lowering material 108) on an emitter tip(e.g., emitter tip 104) of the electron emitter to lower the workfunction of the electron emitter.

Referring, to FIG. 4 , method 400 may include one or more of thefollowing steps 410-430. In step 410, an end portion of an emitter wireor rod is etched to form the emitter tip of the electron emitter. Thebase material of the emitter wire/rod includes atransition-metal-carbide compound or a transition-metal-boride compound.In some embodiments, the emitter wire/rod may be etchedelectrochemically, using any etching method known in the art. In someembodiments, if the emitter tip has already been formed, such as if theelectron emitter is a commercial available emitter who already has anemitter tip, step 410 may be skipped.

In step 420, heat treatment is performed on the electron emitter todesorb contaminants on the surface of the emitter tip. The electronemitter can be heated, for example, by driving a direct current throughthe electron emitter, bombarding the electron emitter with electrons,touching a hot filament directly to the electron emitter, or resistivelyheating the electron emitter by field emission. The present disclosuredoes not limit the method of performing the heat treatment.

In step 430, the emitter tip is coated with a layer of awork-function-lowering material (e.g., work-function-lowering material108). In the disclosed embodiments, the work-function-lowering materialmay include at least one of an oxide compound of zirconium, hafnium,titanium, scandium, yttrium, vanadium, lanthanum, praseodymium,neodymium, samarium, europium, gadolinium, terbium, dysprosium,dysprosium, holmium, erbium, ytterbium, lutetium, or thorium.

Alternatively or additionally, the work-function-lowering material mayinclude at least one of a nitride compound of zirconium, titanium,niobium, scandium, vanadium, or lanthanum.

Alternatively or additionally, the work-function-lowering material mayinclude at least one of an oxynitride compound of zirconium, hafnium,titanium, scandium, yttrium, vanadium, lanthanum, praseodymium,neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium,erbium, ytterbium, lutetium, niobium, or thorium.

FIG. 5 is a flowchart of an exemplary method 500 of preparing anelectron emitter, according to some embodiments of the presentdisclosure. For example, the electron emitter may be electron emitter102 shown in FIG. 2 . The electron emitter may already have an emittertip (e.g., emitter tip 104) suitable for emitting electrons, and method500 may be used to enlarge the size of an apex (e.g., apex 106) of theelectron emitter. The base material of the electron emitter may includetungsten, molybdenum, iridium, rhenium, transition-metal-carbidecompounds, transition-metal-boride compounds, or other kinds of basematerial.

Referring to FIG. 5 , method 500 may include one or more of thefollowing steps 510-550. In step 510, a restraint is applied to theemitter tip. The restraint is used to assist the emitter tip inwithstanding mechanical strain when the emitter tip is under stress, toenhance the mechanical strength of the base material at the emitter tip,to relieve the stress directly applied by the apex enlargement processon the base material of the emitter tip, and/or to keep the emitter tipfrom moving. This way, the apex may be polished without breaking ordamaging the emitter tip. In some embodiments, the restraint may be inthe form of a fixture, a jig, or a clamp holding around, fastened to,attached to, mounted to, or glued to the emitter tip.

FIG. 6 is a schematic diagram illustrating an exemplary fixture jig 109attached to emitter tip 104 of FIG. 2 , according to some embodiments ofthe present disclosure. Referring to FIG. 6 , fixture or jig 109 may bewax, e.g., thermal wax, applied on emitter tip 104. Wax 109 forms alayer around emitter tip 104. The wax layer has a thickness suitable foreffectively relieving stress on the base material of emitter tip 104. Ifemitter tip 104 is pre-coated with work-function lowering material 108,wax 109 may be directly applied on top of work-function loweringmaterial 108. Alternatively, work-function lowering material 108 may befirst removed from emitter tip 104, and wax 109 is subsequently appliedto the base material of emitter tip 104.

Referring back to FIG. 5 , in step 520 of method 500, under therestraint, a planar region with a desirable size may be formed at theapex. Alternatively, if the apex already has a planar region beforemethod 500 is performed, the planar region may be enlarged to reach thedesirable size.

The planar region may be formed or enlarged using various methods. Insome embodiments, the restraint may create enough force to form orenlarge the planar region at the apex. For example, a fixture/jig/clampmay be applied on the emitter tip to form the planar region directly. Inanother embodiment, the emitter tip under the restraint may be polishedto form or enlarge the planar region. The polishing may be performedelectrolytically or mechanically, using any polishing method known inthe art.

Because the restraint (e.g., fixture, jig, clamp, wax, etc.) is appliedto the emitter tip, the emitter tip is capable of sustaining largerstrain or stress without being broken or deformed. As such, a planarregion with a diameter in the range of, e.g., (0.05-10) micrometers, maybe formed at the apex, without damaging the emitter tip.

In step 530, after the planar region with the desirable size is formedapex, the restraint is removed from the emitter tip. For example, if therestraint is in the form of wax, the wax may be melted with heat andwashed with acetone.

In step 540, heat treatment is performed on the electron emitter todesorb contaminants on the surface of the emitter tip. This step issimilar to step 420 of method 400.

In step 550, the emitter tip is coated with a layer of awork-function-lowering material (e.g., work-function-lowering material108). The work-function-lowering material may be selected based on thetype of the base material. For example, when the base material istungsten, molybdenum, iridium, or rhenium, the work-function-loweringmaterial may be oxide, nitride, and carbon compound of zirconium,titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium,beryllium, or lanthanum.

As another example, when the base material is a transition-metal-carbidecompound or a transition-metal-boride compound, thework-function-lowering material may include an oxide compound ofzirconium, hafnium, titanium, scandium, yttrium, vanadium, lanthanum,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, ytterbium, lutetium, or thorium; a nitridecompound of zirconium, titanium, niobium, scandium, vanadium, orlanthanum; and/or an oxynitride compound of zirconium, hafnium,titanium, scandium, yttrium, vanadium, lanthanum, praseodymium,neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium,erbium, ytterbium, lutetium, niobium, or thorium.

FIG. 7 is a flowchart of an exemplary method 700 of preparing anelectron emitter, according to some embodiments of the presentdisclosure. For example, the electron emitter may be electron emitter102 sown in FIG. 2 . Unlike method 600, method 700 may be performed toprepare an electron emitter with a desirable apex size, from an emitterwire or rod without an emitter tip (e.g., emitter tip 104) suitable foremitting electrons. The emitter wire or rod is made of the base materialconstituting the electron emitter to be prepared. Referring to FIG. 7 ,method 700 may include one or more of the following steps 710-760.

In step 710, an end portion of an emitter wire or rod is etched to forman emitter tip of the electron emitter. Step 710 is similar to step 410of method 400.

In step 720, a restraint is applied to the newly formed emitter tip.Step 720 is similar to step 510 of method 500.

In step 730, under the restraint, a planar region with a desirable sizeis form at the apex of the emitter tip. Step 730 is similar to step 520of method 500.

In step 740, after the planar area with the desirable size is formed atthe apex, the restraint is removed from the emitter tip. Step 740 issimilar to step 530 of method 500.

In step 750, heat treatment is performed on the electron emitter todesorb contaminants on the surface of the emitter tip. This step issimilar to step 420 of method 400 and step 540 of method 500.

In step 760, the emitter tip is coated with a layer of awork-function-lowering material (e.g., work-function-lowering material108). This step is similar to step 550 of method 500.

The embodiments may further be described using the following clauses:

-   -   1. An electron emitter comprising:        -   a tip with a planar region having diameter in a range of            approximately (0.05-10) micrometers; and        -   a work-function-lowering material coated on the tip.    -   2. The electron emitter of clause 1, wherein the tip comprises        single crystal tungsten.    -   3. file electron emitter of clause 2, wherein the single crystal        tungsten has a crystal orientation of <100>.    -   4. The electron emitter of clause 1, wherein the tip comprises        at least one of a transition-metal-carbide compound or a        transition-metal-boride compound.    -   5. The electron emitter of clause 4, wherein the        transition-metal-carbide compound is a carbide compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum, or        niobium.    -   6. The electron emitter of clause 4, wherein the        transition-metal-boride compound is a boride compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum,        niobium, or lanthanum.    -   7. The electron emitter of anyone of clauses 4-6, wherein the        work-function-lowering material comprises:        -   at least one of an oxide compound of zirconium, hafnium,            titanium, scandium, yttrium, vanadium, lanthanum,            praseodymium, neodymium, samarium, europium, gadolinium,            terbium, dysprosium, holmium, erbium, ytterbium, lutetium,            or thorium.    -   8. The electron emitter of any one of clauses 4-6, wherein the        work-function-lowering material comprises:        -   at least one of a nitride compound of zirconium, titanium,            niobium, scandium, vanadium, or lanthanum.    -   9. The electron emitter of any one of clauses 4-6, wherein the        work-function-lowering material comprises:        -   at least one of an oxynitride compound of zirconium,            hafnium, titanium, scandium, vanadium, lanthanum,            praseodymium, neodymium, samarium, europium, gadolinium,            terbium, dysprosium, holmium, erbium, ytterbium, lutetium,            niobium, or thorium.    -   10. A thermal field emission cathode, comprising:        -   an emitter comprising:        -   a tip configured to release field emission electrons, the            tip having a planar region with a diameter in a range of            approximately (0.05-10) micrometers; and        -   a work-function-lowing material coated on the tip; and        -   a heating component configured to provide thermal energy to            the emitter.    -   11. The thermal field emission cathode of clause 10, wherein the        tip comprises single crystal tungsten.    -   12. The thermal field emission cathode of clause 11, wherein the        single crystal tungsten has a crystal orientation of <100>.    -   13. The thermal field emission cathode of clause 10, wherein the        tip comprises at least one of a transition-metal-carbide        compound or transition-metal-boride compound.    -   14. The thermal field emission cathode of clause 13, wherein the        transition-metal-carbide compound is a carbide compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum, or        niobium.    -   15. The thermal field emission cathode of clause 13, wherein the        transition-metal-boride compound is a carbide compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum,        niobium, or lanthanum.    -   16. The thermal field emission cathode of any one of clauses        13-15, wherein the work-function-lowering material comprises:        -   at least one of an oxide compound of zirconium, hafnium,            titanium, scandium, yttrium, vanadium, lanthanum,            praseodymium, neodymium, samarium, europium, gadolinium,            terbium, dysprosium, holmium, erbium, ytterbium, lutetium,            or thorium.    -   17. The thermal field emission cathode of any one of clauses        13-15, wherein the work-function-lowering material comprises:        -   at least one of a nitride compound of zirconium, titanium,            niobium, scandium, vanadium, lanthanum.    -   18. The thermal field emission cathode of any one of clauses        13-15, wherein the work-function-lowering material comprises:        -   at least one of an oxynitride compound of zirconium,            hafnium, titanium, scandium, yttrium, vanadium, lanthanum,            praseodymium, neodymium, samarium, europium, gadolinium,            terbium, dysprosium, holmium, erbium, ytterbium, lutetium,            niobium, or thorium.    -   19. The thermal field emission cathode of any one of clauses        10-18, wherein the heating means comprises a filament attached        to the emitter.    -   20. The thermal field emission cathode of any one of clauses        10-19, further comprising:        -   a base;        -   two electrodes embedded into the base; and        -   an electrically conductive wire including two ends and a            central portion, wherein the two ends of the electrically            conductive wire are connected to the two electrodes            respectively, and the emitter is mounted on the central            portion of the electrically conductive wire, the            electrically conductive wire being convex to the emitter at            the central portion.    -   21. A method comprising:        -   applying a restraint to an electron emitter having a tip;        -   under the restraint, forming a planar on the tip; and        -   removing the restraint.    -   22. The method of clause 21, wherein forming the planar region        on the tip comprises:        -   polishing the tip of the electron emitter to form the planar            region.    -   23. The method of any one of clauses 21 and 22, wherein applying        the restraint to the electron emitter comprises:        -   attaching a fixture or a jig to the tip.    -   24. The method of clause 23, wherein forming the planar on the        tip comprises:        -   using the fixture or jig to form the planar region.    -   25. The method of any one of clauses 21-23, wherein applying the        restraint electron emitter comprises:        -   applying wax on the tip.    -   26. The method of any one of clauses 21-25, further comprising:        -   after the restraint is removed, heating the electron            emitter, wherein the heating removes contaminants on the            tip.    -   27. The method of clause 26, further comprising:        -   after the heating, coating a work-function-lowering material            on the tip.    -   28. The method of any one of clauses 21-27, wherein the electron        emitter comprises single crystal tungsten.    -   29. The method of clause 28, wherein the single crystal tungsten        has a crystal orientation of <100>.    -   30. The method of any one of clauses 21-27, wherein the electron        emitter comprises at least one of a transition-metal-carbide        compound or a transition-metal-boride compound.    -   31. The method of clause 30, wherein the        transition-metal-carbide compound is a carbide compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum, or        niobium.    -   32. The method of clause 30, wherein the transition-metal-boride        compound is a boride compound of hafnium, zirconium, tantalum,        titanium, tungsten, molybdenum, niobium, or lanthanum.    -   33. The method of clause 30, further comprising:        -   after the restraint is removed, coating a            work-function-lowering material on the tip, the            work-function-lowering material comprising at least one of            an oxide compound of zirconium, hafnium, titanium, scandium,            yttrium, vanadium, lanthanum, praseodymium, neodymium,            samarium, europium, gadolinium, terbium, dysprosium,            holmium, erbium, ytterbium, lutetium, or thorium,    -   34. The method of clause 30, further comprising:        -   after the restraint is removed, coating a            work-function-lowering material on the tip, the            work-function-lowering material comprising at least one of a            nitride compound of zirconium, titanium, niobium, scandium,            vanadium, or lanthanum.    -   35. The method of clause 30, further comprising:        -   after the restraint is removed, coating a            work-function-lowering material on the tip, the            work-function-lowering material comprising at least one of            an oxynitride compound of zirconium, hafnium, titanium,            scandium, yttrium, vanadium, lanthanum, praseodymium,            neodymium, samarium, europium, gadolinium, terbium,            dysprosium, holmium, erbium, ytterbium, lutetium, niobium,            or thorium.    -   36. The method of any one of clauses 21-35, further comprising:        -   etching an end portion of the electron emitter to form the            tip.    -   37. The method of any one of clauses 21-36, wherein the planar        region of tip has a diameter in a range of approximately        (0.05-10) micrometers.    -   38. A method comprising:        -   coating a work-function-lowering material on a tip of an            electron emitter having a base material, wherein the            work-function-lowering material comprises at least one of:        -   an oxide compound of zirconium, hafnium, titanium, scandium,            yttrium, vanadium, lanthanum, praseodymium, neodymium,            samarium, europium, gadolinium, terbium, dysprosium,            holmium, erbium, ytterbium, lutetium, or thorium;        -   a nitride compound of zirconium, titanium, niobium,            scandium, vanadium, or lanthanum; and        -   an oxynitride compound of zirconium, hafnium, titanium,            scandium, yttrium, vanadium, lanthanum, praseodymium,            neodymium, samarium, europium, gadolinium, terbium,            dysprosium, holmium, erbium, ytterbium, lutetium, niobium,            or thorium.    -   39. The method of clause 38, wherein the base material of the        electron emitter comprises at least one of a        transition-metal-carbide compound or a transition-metal-boride        compound.    -   40. The method of any one of clauses 38 and 39, wherein the        transition-metal-carbide compound is a carbide compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum, or        niobium.    -   41. The method of any one of clauses 38 and 39, wherein the        transition-metal-boride compound is a boride compound of        hafnium, zirconium, tantalum, titanium, tungsten, molybdenum,        niobium, or lanthanum.    -   42. The method of any one of clauses 38-41, further comprising:        -   before coating the work-function-lowering material on the            tip, heating the electron emitter to remove contaminants on            the tip.    -   43. The method of clause 42, further comprising:        -   before heating electron emitter to remove contaminants on            the tip, etching an end portion of the electron emitter to            generate the tip.

It will be appreciated that the present invention is not limited to theexact construction that has been described above and illustrated in theaccompanying drawings, and that various modifications and changes can bemade without departing from the scope thereof. It is intended that thescope of the invention should only be limited by the appended claims.

What is claimed is:
 1. An electron emitter comprising: an emitter tipcomprising a single crystal material, wherein: the emitter tip comprisesan apex with a diameter of less than 10 micrometers, and the singlecrystal material comprises at least one of tungsten, molybdenum,iridium, or rhenium; and a work-function-lowering material coated on theapex, wherein the apex coated with the work-function-lowering materialhas an operating temperature of higher than 700 K.
 2. The emitter ofclaim 1, wherein the single crystal material has at least one of a<100>, <110>, <111>, or <310> crystal orientation.
 3. The emitter ofclaim 2, wherein the apex comprises an emission area substantiallyaligned to a <100> plane of the single crystal material.
 4. The emitterof claim 1, wherein the apex coated with the work-function-loweringmaterial has an operating temperature of less than 1800 K.
 5. Theemitter of claim 1, wherein the emitter is a portion of a thermal fieldemission cathode.
 6. The emitter of claim 1, wherein the diameter of theapex is less than 1 micrometer.
 7. The emitter of claim 1, wherein theapex of the emitter tip comprises a planar region.
 8. The emitter ofclaim 1, wherein the apex comprises an emission area of the emitter tip,the emission area emitting an electron beam.
 9. The emitter of claim 8,wherein the electron beam has an angular intensity in a range ofapproximately 15-400 mA/Sr.
 10. The emitter of claim 8, wherein theelectron beam has an angular intensity in a range of approximately 1-25mA/Sr.
 11. The emitter of claim 1, wherein the apex coated with thework-function-lowering material has a work function in a range ofapproximately 2.3-2.8 ev.
 12. The emitter of claim 1, wherein thework-function-lowering material comprises: at least one of an oxidecompound of zirconium, hafnium, titanium, scandium, yttrium, vanadium,lanthanum, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, ytterbium, lutetium, or thorium,or at least one of a nitride compound of zirconium, titanium, niobium,scandium, vanadium, or lanthanum, or at least one of an oxynitridecompound of zirconium, hafnium, titanium, scandium, yttrium, vanadium,lanthanum, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, ytterbium, lutetium, niobium, orthorium.
 13. A thermal field emission cathode, comprising: an emittercomprising: an emitter tip comprising a single crystal material,wherein: the emitter tip comprises an apex with a diameter of less than10 micrometers, and the single crystal material comprises at least oneof tungsten, molybdenum, iridium, or rhenium; and awork-function-lowering material coated on the apex, wherein the apexcoated with the work-function-lowering material has an operatingtemperature of higher than 700 K; and a heating component configured toprovide thermal energy to the emitter.
 14. The thermal field emissioncathode of claim 13, wherein the apex coated with thework-function-lowering material has a work function in a range ofapproximately 2.3-2.8 ev.
 15. An electron beam tool, comprising: anemitter configured to release field emission electrons, the emittercomprising: an emitter tip comprising a single crystal material,wherein: the emitter tip comprises an apex with a diameter of less than10 micrometers, and the single crystal material comprises at least oneof tungsten, molybdenum, iridium, or rhenium; and awork-function-lowering material coated on the apex, wherein the apexcoated with the work-function-lowering material has an operatingtemperature of higher than 700 K; and a first deflector systemconfigured to split the field emission electrons into an array ofelectron beams.